Process of making tape wound magnetic cores having cube on face orientation

ABSTRACT

A METHOD IS DESCRIBED FOR PRODUCING WOUND MAGNETIC CORES IN WHICH THE CORE MATERIAL IS CHARACTERIZED BY A CUBE ON FACE ORIENTATION. THE STEPS INCLUDE COATING THE MATERIAL AND FORMING THE CORE TO OBTAIN A GIVEN SPACE FACTOR FOLLOWED BY HEAT TREATMENT UNDER CONTROLLED CONDITIONS TO DEVELOP A HIGH DEGREE OF CUBE ON FACE ORIENTATION.

Z-JHP 15, 1971 J. F.-FRITZ E-T L 3,585,085

PROCESS OF MA ING TAPE WOUND MAGNETIC CORES HAVING CUBE 0N FACE ORIENTATION Filed April 2, 1969 3 Sheets-$heet 1 FEGI. -28

I 42 '2 9o 1 LL I a l-Ll F162. 1! v 2 as O E 44 I 70 I 1 l 1 l 1 l 1 l 1 J TENSION-POUNDS/SQUARE INCH WITNESSES INVENTORS Joseph E Frflz 0nd Norman M. Povlik June 15, 1971 INDUCTION KILOGAUSSES Filed April 2, 1969 J. F. FRITZ ETAL 0 3,585,085 PROCESS OF MAKING TAPE WOUND MAGNETIC 'CORES HAVING CUBE ON FACE ORIENTATION 3 Sheets-Sheet 2 5o |OO-- Q 2 3 8 O I I I I I 5 4 s B IO l2 I4 0.

I I 2 I Q I I2 2 I F I63.

s2 8 H v l0- a 8 3 w 54 I/ 5 6- 0 4 4 I I I 0 4 e 8 IO I2 l4 TENSION-POUNDS/SQUARE INCH (x I000) 9-- 70 I 8 & 7..- 6

s-- 72 FIG.5.

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CORE LOSS June 15, 1971 J. rrz EIAL 3,585,085

PROCESS OF MAKING TAPE WOUND MAGNETIC CORES HAVING CUBE ON FACE ORIENTATION Filed April 2, 1969 3 Sheets-Sheet 5 MAGNETIZING FORCE-OERSTEDS O IO 20 3O 4O 5O 22 I I I I v I.

4 I I l l I MAGNETIZING FORCE-OERSTEDS FIG.4.

United States Patent Office 3,585,085 Patented June 15, 1971 3,585,085 PROCESS OF MAKING TAPE WOUND MAGNETIC CORES HAVING CUBE ON FACE ORIENTATION Joseph F. Fritz and Norman M. Pavlik, Pittsburgh, Pa.,

assignors to Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed Apr. 2, 1969, Ser. No. 812,643 Int. Cl. Htllf 3/04, 1/18 U.S. Cl. 148-110 11 Claims ABSTRACT OF THE DISCLOSURE A method is described for producing wound magnetic cores in which the core material is characterized by a cube on face orientation. The steps include coating the material and forming the core to obtain a given space factor followed by heat treatment under controlled conditions to develop a high degree of cube on face orientation.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to magnetic cores and in particular tape wound magnetic cores which are characterized by having a cube on face orientation.

Description of the prior art Within the past ten years a considerable amount of developmental effort has been expended in the production of magnetic materials which are characterized by having the cube on face orientation, that is, the individual grains of the material are aligned in such a way that the cube faces are within the relative plane of the sheet and the cube edges are parallel to the direction of rolling and a direction within the plane of the sheet transverse to the direction of rolling. This is designated as the (110) [001] grain orientation. This orientation is particularly advantageous from the standpoint that there are two easy directions of magnetization, that is, in the rolling direction and the direction transverse to the rolling direction within the plane of the sheet. However, it is quite difficult to obtain such an orientation from a practical standpoint because of the high temperature required in the final anneal which produces said cube on face orientation. At this high temperature adhesion between the layers of the coil will occur if careful provisions are not made for separation. In addition to the problem of interlaminar adhesion the success of the final anneal of the alloy in producing the cube on face orientation is critically dependent upon the annealing atmosphere. In this respect, it has been found necessary to maintain a critical optimum level within a narrow range of sulfur at the surface of the steel during the final anneal by means of the atmosphere surrounding the material. Maintenance of this optimum level can be obtained only when the atmosphree has maximum access to all surfaces of the material being treated. In large tightly wound coils it is difiicult to provide the required access of the atmosphere to the coil midline with the result that many earl-y anneals produce coils with nonuniform development of cube on face orientation.

Moreover, another problem has been encountered in the annealing of large coils of thin gauge strip at the exceedingly high temperatures required. This problem is concerned with the sagging of the coil under its own Weight, resulting in wrinkling of the edges, and general distortion throughout the coil. This condition wastes a considerable portion of the edge of the coil, and causes difficulty in slitting the remainder. As a result it has been found necessary to wind large coils with interlaminar separators in strip form commonly called waster strips. When these waster strips are suitably positioned within the wound coil the nesting laps of the coil are sufficiently separated to provide maximum access of the atmosphere to the coil surface. In addition thereto the size of the coil was considerably reduced in order to minimize sagging which occured during the high temperature annealing. The material was thereafter given a separate application of an interlaminar coating for developing the required resistivity desired between the separate laps of tape wound cores or the individual laminations of stacked cores. However it should also be noted that once the cube on face orientation is obtained in the strip material and thereafter the material is slit and wound into the form of a core or punched to produce laminations which are thereafter assembled into core form, it becomes necessary to stress relief anneal the material in order to reduce the increased losses so as to obtain suitably usable magnetic characteristics.

SUMMARY OF THE INVENTION In order to alleviate the difiiculties experienced with the prior art formation of, for example, tape wound cores, the present method includes employing unannealed, cold rolled strip of the desired finished gauge and width as the starting material for producing the final core product. The cold rolled steel is provided with a non-reactive interlaminar separator coating which has stability at the elevated temperature encountered during the final anneal, is capable of being applied thin enough not to impair the core space factor, has suflicient resistivity after annealing to provide the interlaminar insulation and does not impair the transformation reaction during final anneal to produce the cube on face orientation. The strip material in the cold rolled state having the interlaminar coating applied to at least one side thereof is assembled in the core form either by winding the core at a given rate of speed and under suflicient tension to provide a space factor of between about and about or by stacking the laminations in final core configuration. Thereafter, the tape wound core or the assembled lamination stack is subjected to a heat treatment at a temperature within the range between about 1100 C. and about 1300 C. and in a controlled atmosphere containing a predetermined amount of sulfur until the cube on face orientation is attained within the core material. As thus processed, there is no distortion of the strip due to sagging as occurs with the large coils, and there is no need to provide a temporary interlaminar separator or waster strips in order to have the atmosphere become available to the surface of the material. Notably a stress relief annealing is accomplished concurrently with and during the transformation anneal of the magnetic material thereby eliminating one step, previously required, in its entirety. In addition, there is no necessity for a separate application of an interlaminar coating to provide the necessary interlaminar resistivity in the core after annealing.

In addition it has been found that the cores so produced have improved magnetic loss properties as compared to the cores produced by the use of conversion an neal prior to winding the magnetic core.

The method of the present invention provides a high degree of cube on face orientation of the grains of the material from which the cores are made and said cores exhibit outstanding magnetic characteristics.

It is an object of the present invention to provide a method for producing cores from materials having a cube on face orientation.

Another object of the present invention is to provide a magnetic core having outstanding magnetic characteristics.

A more specific object of the present invention is to provide a process whereby cold rolled strip material can be formed into a core configuration which is thereafter heat 3 treated to provided a cube on face orientation in the materials from which the core is made.

Other objects of the present invention will become apparent to those skilled in the art when read in conjunction with the following description and drawings.

DESCRIPTION OF DRAWINGS FIG. 1 is a schematic illustration for providing a coating to one surface of a tape wound core;

FIG. 2 is a graph of the tension versus the space factor for materials being wound at different speeds into the form of a core;

FIG. 3 is a graph of the Watt loss versus the winding tension for various core winding speeds;

FIG. 4 is a graph of the magnetizing force versus the induction for cores produced by the method of the present invention; and

FIG. 5 is a graph of the core loss versus the induction for cores produced by the process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The method of the present invention contemplates the use of a steel composition containing between about 2% and about 5% by weight of silicon, up to about 0.025% by weight of carbon and the balance essentially iron with residual impurities. This steel is made in any of the well known manners and is preferably cast into ingots, hot rolled in one or more operations and cold rolled to finish gauge in one or more operations usually with one or more anneals including a decarburizing anneal interspersed between the cold rolling operations to provide a final carbon content of 0.005% or less. While any method of producing the sheet steel to which the method of the present invention is applicable is sutficient, it is preferred that the steel be at finish gauge, that is, the steel be in a cold rolled condition having a thickness between about 0.00012 and about 0.012 inch in thickness. It will be appreciated, however, that the degree of final cold rolling should be such as to not unduly limit the ductility of the steel, that is, it should not be in its hardest possible condition resulting from final cold rolling.

The steel having the required composition and the required gauge of the material for forming the desired core is thereafter slit to the required width specification. In this particular instance, it is preferred to work with as narrow a width as possible having due respect to the size of the core to be produced and whether the same is to be formed from a plurality of stacked laminations to form the core or in the form of a toroidal wound core. It will be appreciated that with the thinner gauge materials especially when the same are to be employed in the form of a toroidally wound core it is preferred to have the steel of minimum width dimensions in order to minimize the mass which is subject to the transformation heat treatment, as will be described hereinafter, from the standpoint of reducing the weight of the core so as to prevent any sagging during said transformation heat treatment operation.

The starting material in the cold worked condition may be thereafter surface conditioned if such is required. In this respect, the steel may be subjected to a degreasing operation or any other operation which will remove any extraneous substances which would adversely react with the coating to be applied to the steel for the prevention of interlaminar adhesion during heat treatment as well as the provision of an interlaminar insulating coating.

After the surface of the steel has been prepared for the reception of a coating thereto, it is preferred to provide a coating to the steel which serves both to separate the laminations during the transformation anneal so as to prevent welding thereof and to provide interlaminar electrical insulation in the final core. By thus providing for the separation of the laminations during the transformation anneal and at the same time providing the interlaminar insulation in the final core, it is unnecessary to use interlaminar separators in strip form commonly called waster strips during the winding of the core. In addition, distortion of the strip due to sagging of large coils is completely eliminated since the material will have its finished core width employed as the starting material and as a result thereof, the size and mass of the core is considerably lower than conventional coils thereby substantially eliminating distortion. Since the nesting laps of the tape wound core are in close and tight contact with one another except for the coating, greater rigidity is supplied to the coil turns thereby preventing distortion. Moreover, and in contrast to the prior art, a stress relief annealing is eliminated since the materials in the core will not be disturbed or stressed after they are subjected to the transformation heat treatment.

In addition, there is also the elimination of a separate application of an interlaminar coating to provide the de gree of electrical resistivity sufiicient for interlaminar insulation. Accordingly, the coating which serves to separate the laminations during anneal and provides for the interlaminar insulation in the final core, has high temperature stability, that is, the coating is stable at the elevated temperatures encountered in the final transformation anneal. In addition, it is necessary for said coatnig to be susceptible of application with a coating thickness of sufficient dimensions that the space factor is not unduly impaired. Moreover the coating will have sufficient resistivity after annealing to provide interlaminar insulation and at the same time the coating will not function to impair the transformation process for developing the required cube on face texture orientation in the final core.

While a number of different coatings will function to supply the foregoing enumerated characteristics, it has been found that the use of finely divided aluminum oxide carried in a water slurry containing a predetermined quantity of polyvinyl alcohol is particularly useful in producing the cores employing the method of the present invention. In this respect, it should be noted that the slurries ideally will have a concentration ranging between about 0.7 and about 1.4 pounds of alumina of an average of 370 mesh and finer per gallon of vehicle. The vehicle in this particular instance consists of water into which between about 10 and about 50 grams per gallon of polyvinyl alcohol has been added. The coating slurry is applied to at least one surface of the material which is to be formed into a core although both sides may be coated so long as the space factor of the core is not adversely affected. It is preferred to apply the coating by either dipping or spraying or roller coating. Particular success has been found by dip coating the material and thereafter regulating the coating thickness by means of the speed of transport through the slurry and by wiping the surfaces lightly to control the coating thickness. In the case of toroidally wound cores, it will be appreciated that the slurry density and strip speed will both contribute to the actual coating thickness and the space factor can be controlled by controlling the slurry density and the strip speed. The slurry is dried and the coated strip is wound under tension into a core. Depending upon the desired magnetic properties the space factor may be varied between 70% and about depending upon speed of coating, slurry density and tension. It is also preferred to wind the strip into a core under a predetermined amount of tension. In this respect, it has been noted that the space factor will also be affected by the amount of tension which is employed while winding the strip, In practice it has been found that by regulating the tension of the strip between about 5,000 p.s.i. and about 10,000 p.s.i. excellent control of the space factor of the core can be accomplished. At the same time the magnetic characteristics of the material can be controlled to produce outstanding characteristics particularly core loss when measured at an excitation of 400 hertz and 15 kilogausses. Outstanding results have been obtained where the tension has been maintained within the range 6,000 p.s.i. and 8,000 p.s.i. Where the tension, which is maintained on the strip, is at the lower end of the range the space factor will accordingly be less given the same slurry density and the same strip speeds at which the coating is applied to the strip. This will be more clearly alluded to in the attached drawings. The applied coating is dried by infrared heating, passing the strip through an oven or blowing hot air across the strip.

After the coating has dried, the coated material may be wound directly into the form of a core by toroidally winding the coated material of a preselected width and gauge to provide the desired core configuration. Alternatively, the material may be punched into the form of laminations which are thereafter coated and assembled into a stack of the desired core configuration. Alternatively, the laminations may be first punched, then coated, dried and stacked into core form. In any event, the process of the present invention is effective for producing in the final core the desired degree of freedom from adhesion of the individual laminations during the high temperature annealing treatment, and the final coating will also provide sufficient resistivity to provide the desired degree of interlaminar insulation in the final core yet provide the steel with a maximum of access to the annealing atmosphere to be described more fully.

The assembled cores whether they be of a tape wound configuration or in a stacked lamination configuration are next subjected to a transformation annealing heat treatment in which secondary recrystallization with cube on face grain growth occuring at a temperature within the range between about 1100 C. and about 1300 C. The cores are maintained at this elevated temperature for a time period ranging between about 2 hours and about 24 hours until the required degree of transformation has been completed. During the particular heat treatment involved, it is preferred to supply a predetermined quantity of sulfur to the annealing atmosphere so as to control the sulfur content in the steel thereby fostering the optimum set of conditions for developing a cube on face orienation in the grains that grow. In this respect the atmosphere of the heat treatment is controlled to provide a partial pressure of sulfur sufiicient to provide in at least at the surface of the tape or at each of the lamination surfaces from about 0.00003% to about 0.0006% by weight of sulfur at the time the secondary recrystallization is initiated. It is at this time that the sulfur is controllably removed from the assembled core and thereby fosters the optimum conditions for obtaining the desired cube on face orientation. Such sulfur concentrations can be supplied by the addition of a compound to the annealing atmosphere, for example, hydrogen sulfide in a non-oxidizing gas. As another alternative, the sulfur can be incorporated into the coating and the cores annealed in a non-oxidizing gas. It has been found that a temperature between about 1125 C. and about 1200 C. for a time period of between about 2 hours and about 6 hours having a sulfur content within the range set forth hereinbefore has been effective for developing in at least 70% by volume grains having an orientation in which the cube faces are aligned within ten degrees of the surface of the core laminates and the cube edges of the cube faces are aligned within ten degrees of the rolling direction and within ten degrees of a direction in the plane of the tape and transverse to the rolling direction. Such an alignment gives the cores of the present invention two easy directions of magnetization, that is, in the direction which is parallel to the rolling direction and in the direction which is transverse to the rolling direction but in the same plane as the surface of the core as opposed to the thickness.

In order to more clearly demonstrate the process of the present invention, reference is directed to FIG. 1 which schematically demonstrates the method of producing tape wound cores having the cube on face orientation desired. Referring now to FIG. 1 there is shown a pay off reel shown generally at 10 which pay off reel contains steel 11 having the required final gauge thickness and which has been slit to the desired width. Typically the pay off reel 10 will contain an indeterminant length of the steels 11 in coil form which is thereafter fed around guide roll 12 into a cleaning tank 14 which contains an immersion reel 16 about which the steel 11 is guided. The cleaning tank may contain an agent such as trichloroethylene which is effective for removing any of the rolling lubricate used in producing the steel strip 11 the final gauge thickness. After emerging from the cleaning tank 14, the steel 11 preferably passes through felt wiping pads 20, one of which is disposed on each side of the strip surface. Thereafter the strip 11 is led over guide roll 22 and thereafter down into the coating tank 24 which contains a predetermined level of a slurry 26 which is constantly agitated by means of magnetic stirrer 28. It will be appreciated when the steel 11 emerges from the coating tank 24 the slurry will be deposited on both sides of the strip surface. If only one side of the strip surface is preferred to be coated the apparatus may contain a pair of rods 30 and 32 which have a regulated gap therebetween. Since the strip 11 is tangent for a segment over of the roll 30 and the gap therebetween is regulated by the height of steel 32 the roll 30 is effective for removing the slurry from the underside of the strip 11 as it emerges therefrom. Thereafter the strip 11 is subjected to a bank of heating lamps shown generally at 34 which serve to remove part of the vehicle from the slurry thereby making the coating more adherent to the strip 11. Following the drying operation, the bottom surface of the strip 11 is brought into contact with a felt pad 36 to complete the wiping operation thereby maintaining the desired degree of coating thickness which is thereafter employed through the combination of the tension during the winding of the actual core in order to control the magnetic characteristics and the space factor of the finished core product. The material with the coating on one side thereof can either be wound on a take up mandrel 38 or the core may be directly wound therefrom into the desired configuration. It will be appreciated that where the core is wound into the desired configuration the speed of the strip material through the cleaning tank and coating tank will be regulated so as to obtain the desired degree of coating thickness which will have a pronounced effect on the space factor.

Reference is now directed to FIG. 2 which is a plot of the space factor versus the tension employed through the pay off reel 10 and take up mandrel 38 at various strip speeds through the coating line as set forth hereinfore in FIG. 1. In the curve set forth in FIG. 2 it is to be noted that the steel had a thickness of 0.0014 inch and the tape had a width of inch. A constant slurry density of 1.4 pounds of alumina per gallon of water to which 20 grams of polyvinyl alcohol was added supplied the slurry which was applied to the strip. Referring now to the drawings curve 40 illustrates the variation in the space factor versus the tension where no coating was placed on the strip, curve 42 illustrates the space factor where the strip speed was maintained at 40 inches per minute, curve 44 a strip speed of 90 inches per minute and curve 46 a strip speed of inches per minute through the line. It is noted that as the tension on the strip was increased a mark increase was noted in the space factor obtained. Thus, where the strip speed was minimum and the tension maintained at about 8,000 p.s.i., a space factor of 90% was obtained on a coated strip when wound into the finished core. Where, however, the strip speed is increased it is noted that the space factor corresponding decreases for the same amount of tension which is applied thereto during winding of the tape core. The effect of the space factor variations by means of varying tension on the magnetic char acteristics is more clearly illustrated by reference to FIG. 3 which is a plot of the core loss at 400 hertz at an intensity of 15 kilogausses versus the tension for the same material.

In FIG. 3 the curve 50 illustrates the effect of tension on the 400 hertz watt loss and corresponds to the winding speed of curve 42 of FIG. 2, that is, the same strip speed and slurry density. Thus as the tension increases for the same strip speeds higher space factors are obtained only at a great sacrifice in the magnetic characteristics exhibited by the finished core. Curve 52 of FIG. 3 demonstrates the effect on the core loss when the strip speed is at 90 inches per minute and accordingly corresponds with curve 44 of FIG. 2 and curve 54 demonstrates the watt loss properties employing a strip speed of 150 inches per minute and thus corresponds to the curve 46 of FIG. 2. Thus, while there will be some loss in the space factor as a result of using higher strip speeds with a constant slurry density, nonetheless the higher strip speeds are also associated with lower core loss and as a result this reflects in the character of the coating thickness of the materials thus employed.

Referring now to FIG. 4, there is illustrated a normal magnetization curve for materials which were treated in accordance with the process of the present invention versus the prior art practice which included the subjection of the material to a transformation anneal followed by the application of a coating, winding the material into the desired core configuration stress relief annealing and thereafter testing the core so produced. Curve 60 illustrates the normal magnetization curve for the material processed by the method of the present invention and curve 62 the material which is processed by means of the prior art processing. It is noted that the complete magnetization curve is set forth on two sets of coordinates and accordingly, the curve 60 is the continuation of the curve 60 using the upper coordinates in the plot and curve 62 is a continuation of the curve 62. It will be seen that at operating inductions of about 15 kilogausses there is no significant deviation between the curves the same being at most about less than 1 oersted. While some divergence is noted when the materials attain or approach saturation the difference is relatively minor and insignificant.

Referring to FIG. 5 substantially the same results are obtained when comparing curves 70 and 72. Curve 70 is a plot of the induction versus the core loss for material processed according to the present invention and curve 72 is the plot of induction versus core loss of cores formed from prior art practices. From FIG. 5 it will be noted that at the same operating inductions substantially lower watt losses are encountered in the materials processed by the method of the present invention until the operating induction approaches about 13 kilogausses. Thereafter, the divergence in the curves as the induction approaches saturation shows no significant material difference in the watt loss exhibited by these materials. Accordingly, it is clear that the method of the present invention produces an outstanding combination of magnetic characteristics when steel is treated in accordance with the method of the present invention.

From the foregoing it will be apparent that the method of the present invention is applicable for producing material in tape wound core form which eliminates a number of the prior art processing steps yet attains a transformation in the core such that the core will exhibit two easy directions of magnetization. This results from the fact that the processing parameters set forth hereinbefore are effective for providing the optimum set of conditions for developing a cube on face orientation to at least 70% of the grains by volume. This cube on face orientation is effective for producing outstanding magnetic characteristics as clearly set forth by the data contained in the attached drawings. Moreover, by transforming the steel in core form great savings have been effected in the overall use of the steel as well as minimizing some of the problems associated with the prior art, namely, the sagging of heavy coils during the high temperature anneal. In addition thereto, the annealing atmosphere is more freely accessible to the over all surface of the materials being heat treated thereby resulting in a more uniform transformation product resulting in a greater degree of cube on face orientation being attained in the final product. Moreover, by the elimination of the necessity of the separate application of an interlaminar insulating coating and the necessity for stress relief annealing of the material following the slitting and winding of the material into core form results in great economies to the core manufacture. Since all of these advantages are obtained in such a manner that the magnetic characteristics can still be regulated by the proper interrelation of the various parameters of the process of the present invention, it becomes clear that a greater degree of flexibility can be attained in the manufacture of cores whether the same or manufactured from stacked laminations or in wound tapes.

It will be appreciated that various modifications and variations may be employed by those skilled in the art without departing from the underlying spirit and scope of the present invention. Nonetheless, it is considered that the description contained herein is exemplary only and such modifications and variations are intended to be covered by the appended claims.

We claim:

1. In the method of producing cores having a cube on face orientation and in which at least about by volume of the grains have an orientation in which the cube faces are aligned within 10 of the surface of the members forming the core and the cube edges of the cube faces are aligned within 10 of the rolling direction and within 10 of the direction in the plane of the material and transverse to the rolling direction, the steps comprising, selecting a cold rolled steel of finish gauge thickness within the range between about 0.00012 and 0.012 inch and of a composition containing between about 2% and about 5%, by weight of silicon, up to 0.025% by weight of carbon and the balance essentially iron with residual impurities, applying to at least one surface of the selected material an insulating coating having stability at a temperature of at least 1200 C. while imparting sufficient resistivity to provide interlaminar electrical insulation after heat treatment, assembling the coated material into the form of a core while controlling the coating thickness and tension of said finish gauge material to provide a space factor within the range between 70% and and thereafter annealing the core at a temperature within the range between 1100" C. and 1300 C. in a nonoxidizing atmosphere containing a partial pressure of sulfur sufficient to provide in at least at the surface of the members forming the core from 0.0003% to 0.0006% of sulfur at the time secondary recrystallization is initiated to transform the grains of the core by secondary recrystallization to a cube on face orientation.

2. The method of claim 1 in which the insulating coating comprises a slurry of alumina suspended in a vehicle of water and polyvinyl alcohol.

3. The method of claim 2 in which the slurry density is maintained within the range between about 0.7 and 1.4 pounds per gallon of vehicle.

4. The method of claim 1 in which the insulating coating is applied continuously and the material is wound into the form of a toroidally wound core at a lineal speed of between about 50 and about 300 inches per minute.

5. The method of claim 4 in which the material is maintained in tension during winding into core form, said tension applying a force to the material of between about 5,000 p.s.i. and 10,000 p.s.i.

6. The method of claim 1 in which said transformation during annealing takes place in a magnetic field.

7. In the method of producing cores having a cube on face orientation and in which about 70% by volume of the grains have an orientation in which the cube faces are aligned within 10 of the surface of the members forming the core and the cube edges of the cube faces are aligned within 10 of the rolling direction and within of the direction in the plane of the material and transverse to the rolling direction, the steps comprising, selecting a cold rolled steel of finish gauge thickness within the range between about 0.00012 and 0.002 inch and of a composition containing between about 3% and about 3.5%, by weight of silicon, up to 0.025% by weight of carbon and the balance essentialy iron with residual impurities, applying to at least one surface of the selected material an insulating coating of alumina suspended in a vehicle of Water and polyvinyl alcohol, said coating having a density of between 0.7 and 1.4 pounds of alumina per gallon of vehicle assembly the coated material into the form of a core while controlling the coating thickness and tension of said finish gauge material to provide a space factor within the range between 70% and 90% and thereafter annealing the core at a temperature within the range between 1125 C. and 1200 C. for a time period of between about 2 and 34 hours in a non-oxidizing atmosphere containing a partial pressure of sulfur sufficient to provide in at least at the surface of the members forming the core from 0.00003% to 0.0006% of sulfur at the time secondary recrystallization is initiated to transform the grains of the core by secondary recrystallization to a cube on face orientation.

8. The method of claim 7 in which the insulating coating is applied continuously and the material is wound into the form of a toroidally wound core at a lineal speed of between about 50 and about 300 inches per minute.

9. The method of claim 8 in which the material is maintained in tension during winding into core form, said tension applying a force to the material of between about 6,000 p.s.i. and 8,000 p.s.i.

10. The method of claim 1 in which said transformation during annealing takes place in a magnetic field.

11. In the method of producing tape wound cores having a cube on face orientation and in which about 70% by volume of the grains have an orientation in which the cube faces are aligned with 10 of the surface of the tape and the cubic edges of the cube faces are aligned within 10 of the rolling direction and within 10 of the direction in the plane of the tape and transverse to the rolling direction, the steps comprising, selecting a cold rolled steel of predetermined width, finish gauge thick ness within the range between about 0.00012 and 0.002 inch and of a composition containing between about 3% and about 3.5%, by weight of silicon, up to 0.025% by weight of carbon and the balance essentially iron with residual impurities cleaning the surface of the tape, applying to at least one surface of the tape an insulating coating at a rate between about and about 300 lineal inches per minute, said coating comprising a slurry of alumina in a vehicle of water and polyvinyl alcohol, said slurry having a density of between about 0.7 and about 1.4 pounds of alumina per gallon of vehicle, winding said coated tape into the form of a core while applying tension to the tape within the range between 6,000 p.s.i. and 8,000 p.s.i. to provide a space factor within the range between and and thereafter annealing the core at a temperature within the range 1125 C. and 1200 C. for a time period of between about 2 hours and 6 hours in a non-oxidizing atmosphere containing a partial pressure of the members forming the core from 0.00003% to 0.0006% of sulfur at the time secondary recrystallization to a cube on face orientation.

References Cited UNITED STATES PATENTS 2,920,296 1/ 1960 Neurath 148113UX 2,980,561 4/1961 Ford et al. 1481 12X 3,152,930 10/1964 Foster 148113 3,240,638 3/1966 Wiener et al. 148--111X 3,282,747 11/1966 Foster et al. 148-113 3,421,925 1/1969 Hair et al. 1481l2X L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. X.R. 148-ll2, 113 

